AS the site of the secret workshops of the Manhattan Project, Los Alamos, N.M., is known to history as ''the atomic city'' -- the place where scientists harnessed nuclear power, building the bomb that destroyed Hiroshima. The Cerro Grande fire, which burned on last week, was a reminder of how absurd it is to believe that mankind can ever harness a force of nature. Los Alamos learned to tap the power of nuclear energy. But it was brought to a standstill by a different kind of chain reaction: ordinary fire.

For decades the Los Alamos National Laboratory has steeled itself against the possibility of a nuclear missile strike. It has guarded against terrorists, spies and computer hackers. But the threat that turned out to be most immediate was disquietingly familiar: the tens of thousands of acres of neighboring Ponderosa forests that crowd the Pajarito plateau and the adjacent Jemez mountains, and which exploded, as had long been feared, into a firestorm.

A small ''controlled burn'' -- in Cerro Grande's aftermath, the very term has become suspect -- was lit May 4 in a remote corner of Bandelier National Monument, a canyon wilderness sheltering cliff dwellings and stone-walled ruins of the ancient people called Anasazi. The idea was to clear encroaching pines and other combustible debris from an overgrown mountain meadow, making it safer from fire. But winds unexpectedly pushed the flames beyond the prescribed perimeter, and ultimately beyond imagination and control.

Within hours the fire had overwhelmed the small team assigned to it. A ''back burn'' lit three days later to control the first blaze -- literally fighting fire with fire -- raced out of control and began sweeping toward Los Alamos. Before it was over, more than 47,000 acres of forest -- roughly three times the size of Manhattan -- had been consumed in the biggest fire in the history of New Mexico, a dry, heavily timbered state that has become accustomed to mammoth conflagrations. Los Alamos was evacuated, its residents returning to find more than 200 residences destroyed. Late last week, the fire was on its way to being contained, but firefighters predicted that it would continue burning for many days.

Theorists at the lab who study complex systems like whirlpools and thunderstorms -- not everyone there designs bombs -- often refer to a phenomenon called ''sensitive dependence on initial conditions.'' Chaos, for short. Tiny actions, the theory goes, can have enormous, unforeseeable consequences. The idea has entered the culture of popular science as ''the butterfly effect.'' A mythological insect flapping its wings in Rio de Janiero causes a hurricane off the coast of Louisiana. A small fire routinely lit to clear a few hundred acres of tinder closes down a nuclear research center.

Foresters try to plan their intentional burns like lab technicians preparing for an experiment. They gather data -- the moisture content of the wood, the wind speed, the relative humidity and temperature of the air. They plug the factors into a computer model -- those assemblages of software that spin the comforting illusion that nature can be contained by numbers. They take into account the lay of the land, whether natural features like cliffs and streams will help enclose the burning. And they consider how likely it is that the plan could go awry (a factor crucial in the postmortem of Cerro Grande) and what it would take to set it right again.

But one can never know how the butterfly will flap its wings. A sudden wind or a moment of inattention can make the calculations irrelevant. Whipped by gales like the ones that overtook Cerro Grande, the fire creates its own weather, becoming so large that, like a hurricane, it is given its own name.

RESULTS from a preliminary Department of Interior investigation laid the blame not on the precision of the data or the accuracy of the calculations but on matters of human judgment. The fact that strong prevailing winds were on the horizon was left out of a National Weather Service report, and no one asked for the missing data. Compounding this oversight, planners at Bandelier underestimated the potential complexity of the situation -- how many people and how much equipment they would need if the fire didn't behave as it was supposed to.

There was a list of other deviations as well, the seriousness of which will be evaluated as investigators try to determine which butterflies' flapping did most to spread the flames. Taken together they tell a familiar story: there was a gulf between the way the fire played out in the calculations of the planners and the way it unfolded in the unforgiving real world.

Some politicians and homeowners were eager to have a specific villain to blame, but the culpability was far more diffuse. Large-scale cattle grazing and the clear-cutting of timber in the 19th century, followed by decades of dutiful suppression of natural wildfires, have conspired to produce thickets of highly combustible pines all over the region. A 1998 National Forest Service report warned that during the next five years there was a 30 percent chance of a large fire striking the woods around Los Alamos, sparked perhaps by one of the numerous summer lightning storms. And a Department of Energy report published last year concluded that ''a major fire moving up to the edge of Los Alamos National Laboratory is not only credible but likely.''

The same danger threatens forests all over the West. At the same time as Cerro Grande, two major fires in southern New Mexico were burning, one ignited by a downed power line, the other by a campfire. Two devastating fires had come dangerously close to Los Alamos in 1977 and 1996. After the second one, tinder was cleared to create firebreaks. But protecting the town from something like Cerro Grande would have taken a huge public works program -- a kind of Manhattan Project involving saws and axes. Whether that would have been as costly as the estimated billion dollars in damage the fire has caused is now a matter of historical curiosity.

Because the fire was burning on the grounds of a nuclear lab, people downwind have naturally feared contaminated smoke. But air monitoring indicates that the most significant radiation and toxins released were the naturally occurring kind, as inevitably happens when millions of sap-filled trees are set afire. Even when it comes to pollution, nature again outdoes the efforts of mankind.

Having been besieged by fire, Los Alamos will soon face another elemental threat: water, when the annual rainy season begins in late June or early July. Repelled by the ash-covered ground, and undeterred by burnt-over vegetation, water is expected to sweep down the canyons toward the Rio Grande, causing large-scale erosion. How significantly trace contaminants in the soil -- both from the lab and released naturally by the fire -- will add to the problem is uncertain. But there is enough to fear from the water and the silt itself.

Archeologists still debate what drove the Anasazi from their stone settlements. Drought, warfare, boredom -- no one really knows. The abandonment of Los Alamos was brief and the ruins will be rebuilt. But it's hard to escape the conclusion that the modern settlers were driven off the plateau by the cumulative force of their leaders' own actions and indecisions. And by the illusion that they were in control.